1. Field of the Invention
The present invention relates to an optical switch employed for switching optical paths in an optical communication system and a method of manufacturing the same.
2. Description of the Background Art
A conventional optical switch 90 will be described with reference to FIG. 21. This optical switch 90 is one shown in TuM1 (J. E. Fouquet, “Compact optical cross-connect switch based on total internal reflection in a fluid-containing planner lightwave circuit”) at OFC 2000 (Optical Fiber communication Conference, Mar. 7, 2000). Optical switch 90 is formed by a silica planar optical circuit substrate 51, where a two-dimensional optical circuit is formed by providing a rectangular optical waveguide having a slightly high refractive index within silica planar optical circuit substrate 51. Generally, silica planar optical circuit substrate 51 having a refractive index of about 1.5 is employed, while optical waveguide 91 portion is made of a material with a refractive index that is higher by about 1%. The portion of optical waveguide 91 is in general also referred to as a “core.” A trench 52 is formed such that it traverses a position that partially overlaps with a crosspoint of optical waveguide 91, and trench 52 is filled with refractive index-matching oil 56. Refractive index-matching oil 56 is oil having a refractive index equal to that of optical waveguide 91.
An operation of optical switch 90 will be described. In optical switch 90, a bubble generating mechanism, not shown, is provided which can generate a bubble 53 and also make bubble 53 disappear in the liquid of refractive index-matching oil 56 at a crosspoint of optical waveguide 91. For the bubble generating mechanism, as that used for a head of bubble-jet printer may be employed.
When bubble 53 is generated at a crosspoint, a light that enters into the crosspoint is totally reflected by a surface of bubble 53, while when no bubble 53 exists, the light travels in a straight line because optical waveguide 91 and refractive index-matching oil 56 have an equal refractive index. By utilizing this property, an optical path can be switched between two states of reflection/straight travel by generation/disappearance of bubble 53.
In optical switch 90 formed in the above-described manner, accuracy of an etching process requires a width of trench 52 to be at least about 15 μm. In addition, there is a problem of optical loss of at least 0.07 dB per crosspoint. On the other hand, optical loss L [dB] of optical switch 100 as a whole is given by the following formula:L=2C+(m−1)T+(n−1)T+R,    where m is a number of input ports,    n is a number of output ports,    C is a loss [in dB] upon entry of a light into an optical switch from an optical fiber and during travel through an optical waveguide to an active area which is a crosspoint,    T is a loss [in dB] upon traversing one trench and during transmission through a section of a short optical waveguide between two crosspoints, and    R is a loss [in dB] for reflection upon a sidewall of an empty trench and transmission through a section of a short optical waveguide between two crosspoints.
For instance, when configuring a large-scale 1000×1000 optical switch, specific numerical values substituted into the above formula give L=2×0.25+(1000−1)×0.07+(1000−1)×0.07+2.1=142.46 dB. Thus, even with an ideal produced optical switch, optical loss of 142.26 dB would occur. It is necessary to limit optical loss L to 10 dB or below in order for an optical switch to function without degrading signal quality. In this manner, there is a disadvantage in that a larger scale than about 32×32 is difficult to form with such type of configuration when optical loss is considered.
Moreover, silica planar optical circuit substrate 51 is produced by a device similar to that which produces a semiconductor so that a large optical switch would disadvantageously become extremely expensive to produce. Further, since bubble 53 is produced and utilized each time it is needed in refractive index-matching oil 56, there is a problem of optical switching malfunction occurring when the generated bubble 53 is too small or when the bubble is generated out of position. Furthermore, depending on the condition of refractive index-matching oil 56, local absorption of light would take place with a small globule of refractive index-matching oil 56 such that an optical path in its periphery would disadvantageously burn due to the energy of a signal light.